Study protocol for a randomized controlled trial of a self-determination theory-based psychoeducational intervention...

Physical inactivity, maladaptive exercise motives, and body image concerns are highly prevalent among young adult women and are associated with reduced psychological well-being and increased risk for disordered eating. Although previous interventions have addressed physical activity or body image separately, few have simultaneously targeted motivational quality and body-related perceptions within an integrated theoretical framework. This study protocol describes a randomized controlled trial designed to evaluate the effectiveness of a psychoeducational intervention based on Self-Determination Theory and using functionality-oriented body image strategies. The aim is to enhance autonomous motivation for exercise and improve psychological well-being among female university students in Health Sciences. Participants aged 18-25 years will be randomly allocated (1:1) to an experimental group or a wait-list control group following baseline assessment. Outcome measures will be collected at baseline (T0), post-intervention (T1), and six-month follow-up (T2). The intervention consists of six face-to-face group sessions that include autonomy-supportive strategies, competence-building activities, and functionality-based body image content emphasizing body capabilities rather than appearance. The primary outcome is autonomous motivation for exercise, assessed using the Behavioral Regulation in Exercise Questionnaire-3 and operationalized through the Relative Autonomy Index at six-month follow-up. Secondary outcomes include satisfaction and frustration of basic psychological needs, physical activity levels, positive body image, and risk of eating disorders. Data will be analyzed using mixed model for repeated measures under an intention-to-treat framework to account for repeated measures and missing data. Mediation analyses will examine whether changes in autonomous motivation explain the effects of the intervention on physical activity, positive body image, and eating disorder risk. This study will provide evidence on the effectiveness of an integrated, theory-based intervention targeting motivation and body image in young adult women. The trial is registered at ClinicalTrials.gov (Identifier: NCT07506343).